This invention relates to an electric arc furnace roof cover having provision for liquid cooling therein and further having means for dramatically reducing eddy currents in the portion of the cover adjacent the arc furnace electrodes which protrude therethrough.
Liquid cooled roofs or covers for electric arc furnaces are known from U.S. Pat. Nos. 1,922,312; 4,197,422; 4,273,949; 4,443,880; and others. Each of these patents teaches a construction which promotes eddy currents within the roof.
Water cooled electric furnace roofs or covers are known from Buhler et al U.S. Pat. No. 4,443,880, which teaches a unitary furnace cover, having parallel cooling pipes generally vertical to the furnace tipping direction and having a specified spacing between the cooling pipes. Mannsfield U.S. Pat. No. 1,922,312 teaches a cover having a plurality of sections 21 separated from each other by insulating walls 23. Mannsfield recognizes that there are induction losses in the cover, and utilizes a non-magnetic portion 31 in an attempt to reduce such losses.
It has long been desired to provide a liquid cooled furnace roof for electric furnaces which is compatible with liquid cooled furnace wall panels. It is particularly desirable to utilize a cooled furnace cover which includes provision for electrical and thermal insulation on its underside.
In the situation in which three electrodes protrude through a single hole in a roof, if two electrodes should happen to touch the side of the hole at the same time they will create an arc, current will flow between them, and the arcing will damage the panel, usually by creating one or more holes in the panel, resulting in the loss of all of the cooling fluid (water) through such hole. Water is dangerous in a furnace, and its presence can lead to an explosion.
In the case where a roof has three holes in it, an electrode positioned in each hole, the current passing downwardly through the electrode sets up an induction current around the hole, because the metal around each electrode is completely conductive, being completely connected to itself all the way around the electrode. Providing three separate tunnels as a portion of the roof without a complete ring of metal around any one electrode, breaks up the tendency to develop induction current in the roof panels.
The passing of induction current through the roof around the electrodes can lead to arcing, to over heating of the metal in the panel, and ultimately to loss or drain of energy. The tendency for induction currents to be set up increases by the size of the transformer. Small, low power furnaces have a lesser tendency to create induction currents.
The present invention avoids the creation of induction currents in the roof by electrically separating the potential from all three electrodes.